Field of the Invention
The invention relates to electron impact ion sources for use in mass spectrometers, particularly in benchtop mass spectrometers, e.g. gas-chromatograph/mass-spectrometers (GCMS).
Description of the Related Art
Usually, gas-chromatograph/mass-spectrometer instruments use electron impact (EI) sources to create ions. In the most common prior art (see FIG. 1) the sample is vaporized in the GC and introduced into the source where the sample molecules are effusing from the end of a GC column (41) and bounce on the inside walls of the ionization chamber (40) creating a transient local pressure before they diffuse through the source openings and are pumped away. The EI source uses a filament assembly (42) with a straight filament that generates electrons, which are accelerated to typically seventy electron volts toward the ionization region where they collide with sample molecules and ionize. The electrons can be guided by a magnet assembly comprising two magnets (46) and (47) and a magnetic yoke (48). The ions are extracted from the ion source housing (40) by apertured electrodes (44) and form an ion beam (45). The source operates in high vacuum, at pressures lower than one Pascal, such as 10−2 Pascal or even less, so that the ionization occurs under conditions where the mean free path is larger than typical dimensions of the source.
Electron impact cross sections are very small and in a typical EI source several measures are commonly taken to improve ionization efficiency.
By way of example, U.S. Pat. No. 9,117,617 B2 (Agilent Technologies, Inc., Santa Clara, Calif. (US), Charles William Russ, IV, Harry F. Prest, Jeffrey T. Kernan, “Axial Magnetic Ion Source and related Ionization Methods”; filed Jun. 24, 2013) uses an axial alignment of the electron path and ion extraction path which increases the ion extraction efficiency of the EI source. Nevertheless, the ionization area is still limited to a narrow confined space along the axis of the source while the sample molecules are introduced at right angle to the electron path and spread through the entire source volume. So the ionization efficiency is still relatively small.
Another prior art technique, described in U.S. Pat. No. 6,617,771 B2 (Aviv Amirav “Electron Ionization Ion Source”, filed Jan. 24, 2002), introduces the sample into the source as a confined supersonic jet through a nozzle-skimmer arrangement, followed by cross beam electron ionization (see FIG. 2). One advantage is that the sample is now confined into a narrow jet volume. Another advantage is that the sample molecules do not hit any source walls, thus eliminating some disadvantages of the generic EI source as exemplarily illustrated in FIG. 1. Electron ionization is achieved by an electron curtain coming from a long filament oriented parallel with the neutral sample gas jet. The disadvantage is poor ionization efficiency due to the poor electron beam confinement and single pass of the emitted electrons through the sample jet. As a result, very large electron emission currents need to be used which leads to filament deformation in time and heat management complications.
Yet another prior art technique is presented in FIG. 3, schematically depicting the ion source by M. DeKieviet et al. “Design and performance of a highly efficient mass spectrometer for molecular beams”; Review of Scientific Instruments, May 2000; vol. 71, No. 5. DeKieviet et al. also introduce the sample as a confined gas jet (51) followed by electron impact ionization, but the electron beam (56) from a ring filament assembly (50) is focused and aligned with the jet area by a magnetic field (55) generated by a solenoid magnet (52) downstream of the ring filament. The electrons are generated off-axis, within the fringe field of a solenoidal magnet, and then accelerated toward the axis of the source where the sample jet (51) flows. Acceleration occurs along the magnetic field lines such that the electrons (56) spiral there-around at radii becoming ever smaller as they get closer to the axis where the field is denser.
This configuration has some advantages: it confines the neutral sample into the area of a jet and then it confines the electrons into the same area using the solenoidal magnetic field such that ionization and ion extraction can have high efficiency. One disadvantage would be the large current required for the solenoid to generate the necessary strong magnetic field, which requires significant cooling and limits the application of this source to large-dimension, high power instruments. This basically excludes the use of this source in the typical GCMS where benchtop instruments are the norm and actually largely demanded by customers. Another disadvantage would be the creation of a sort of “magnetic trap” on the gas jet path inside the solenoid body where a large number of electrons could accumulate over time ultimately leading to space charge issues.
In view of the foregoing, there is still a need for a small, high efficiency electron impact ion source for mass spectrometry, particularly for GCMS instrumentation.